Structural, Electronic, and Magnetic Properties of NiGa₂O₄

Abstract

In this study, NiGa₂O₄ phase is prepared using a sonication-assisted hydrothermal method. Rietveld-fitted x-ray diffraction results reveal the formation of a single-phase compound with average crystallite size of ~32.5 nm. UV-visible absorption spectroscopy results show bandgap energy of 4.1 eV. X-ray absorption near-edge structural analysis confirms the presence of Ga³⁺ and Ni²⁺ ions in Ga₂NiO₄. The magnetic properties of NiGa₂O₄ phase are studied at two temperatures (i.e., 50 K and 300 K), and the NiGa₂O₄ phase is found to be paramagnetic. The origin of magnetism in the NiGa₂O₄ phase is mechanistically discussed by considering the plausible magnetic interactions among the constituent ions in the compound.

Introduction

The AB₂O₄ spinel oxides are attractive compounds exhibiting diverse properties because of multifaceted interactions among the constituent cations of different sizes and variable valence states. AB₂O₄ compounds have also been reported with catalytic, light emission, gas-sensing, and magnetic properties.^1,2,3,4,5,6 Based on the distribution of cations at octahedral and tetrahedral sites, the AB₂O₄ spinel oxides can be divided into two broad categories, and the AB₂O₄ formula unit can be written in terms of the degree of inversion parameter (x) as {A_1−xB_x}[B_2−xA_x]O₄. In this formula unit, the cations in the curly brackets and square brackets represent the tetrahedral and octahedral sites, respectively.^7,8 For the normal spinels (with x = 0, in the above formula unit), the cubic symmetry (Fd-3m) provides octahedral sites to the B element and tetrahedral sites to the A element. On the other hand, the inverse spinels (with x = 1) consist of all of the A cations and half of the B cations at octahedral sites, and the remaining half of the B cations occupy the tetrahedral sites. Other hybrid spinels can also be prepared with the 0 < x < 1 configuration; however, they are high-temperature phases and known as disordered-dual spinel phases.^9,10 Among all AB₂O₄ compounds, gallate spinels have attracted particular attention.^11,12 Some of the transition metal-containing MgGa₂O₄ spinels exhibit absorption and luminescence properties useful for optoelectronic devices.¹³ Nickel gallates have also been found to be a potential catalyst for NO reduction as well as for the photocatalytic dissociation of water.^14,15 NiGa₂O₄ has also found application in supercapacitors, gas sensors, and glucose sensors.^16,17,18

Various synthesis approaches have been established to grow nickel gallates;^19,20 however, the distribution of constituent cations (at tetrahedral/octahedral sites) largely depends on the operating temperatures and other synthesis protocols. A slight variation in the site occupancy of an element (driven by synthesis protocols) may facilitate diverse hybridization of frontier orbitals and, consequently, a variety of electronic structural and magnetic properties in the compounds. For the case of the NiGa₂O₄ compound, the relationship between the electronic structure and magnetic properties is not significantly reported in the literature. In this study, NiGa₂O₄ nanocrystals are prepared using a sonication-assisted hydrothermal method. The crystal structure and lattice parameters of NiGa₂O₄ are investigated using x-ray diffraction (XRD) and Rietveld refinement. Hybridization of frontier orbitals and electronic structural properties are studied using x-ray absorption near-edge structure (XANES) spectroscopy at the O K-, Ni K-, and Ga K-edge. The low-temperature (50 K) and room-temperature magnetic properties of NiGa₂O₄ nanoparticles are also investigated and mechanistically discussed by considering the magnetic interactions among the constituent cations.

Experimental

The pure NiGa₂O₄ phase was synthesized by the wet chemical method. All the reagents used were of analytical grade without further purification. One gram of Ga(NO₃)₃ and 1 g of C₄H₆NiO₄.4H₂O (nickel (II) acetate tetrahydrate) were separately dissolved in 200 ml ethanol and stirred (100 rpm) for 1 h. The solutions were poured into another beaker and stirred (200 rpm) for 30 min followed by sonication for 30 min. Next, 10 mol ethyl glycol (20 ml) was added to the above-prepared solution, and the final dense solution was heated at 80°C in an oven. The as-dried powder was annealed at 1150°C for 4 hours. The cooling steps were kept the same as the temperature gradient heating steps (with a heating/cooling rate of 15°C/min). XRD measurements were performed using Cu Kα radiation with a wavelength of 1.5418 Å. The Bruker D8 Advance diffractometer, operated at 40 kV accelerating voltage and 40 mA tube current, was used in this work. During data collection, the scan speed was 1°/min in steps of 0.02°. The XRD data were analyzed with Rietveld refinement using FullProf software. O K-edge XANES spectra were collected in total electron yield (TEY) mode at the soft x-ray beamline (10D-XAS-KIST) of the Pohang Accelerator Laboratory (PAL), South Korea. The photon energy resolution of this beamline was better than 0.6 eV (at the O K-edge). Ni K-edge and Ga K-edge XANES spectra were collected from the x-ray scattering beamline (1D XRS KIST-PAL) of PAL, South Korea. The data collection procedure and uses of gas mixtures and typical procedures for the background removal and normalization are provided elsewhere.²¹ Low-temperature (50 K) and room-temperature (300 K) magnetization measurements were performed using the commercial Quantum Design Physical Property Measurement System (PPMS).

Results and Discussion

Figure 1 shows the XRD pattern for the NiGa₂O₄ compound along with the Rietveld refinement curve, Bragg’s peak positions, and difference line. The XRD pattern was refined with a cubic unit cell (space group Fd-3m). Good agreement is observed between the calculated and experimental XRD patterns, evidenced by the almost flat, straight difference line in Fig. 1. There are no traces of other phases (e.g., Ga₂O₃, NiO) in the XRD patterns, indicating the single-phase formation of NiGa₂O₄ by applying the given synthesis protocols. The calculated lattice parameters, Wyckoff positions, and other fitting parameters are listed in Table I. A comparison of lattice parameters, space group, and Wyckoff positions of the NiGa₂O₄, Ga₃O_2, and NiO²² compounds is also tabulated in Table I. The distinct difference in the lattice parameters and Wyckoff positions of the three compounds indicates dissimilarity in their crystal structures and space groups. The average crystallite size of NiGa₂O₄ is calculated using the Scherrer relation D = 0.9λ/(βcosθ), where β is the full width at half-maximum of the peaks. The estimated crystallite size is ~32.5 nm.

Fig. 1

Rietveld-refined XRD patterns of NiGa₂O₄.

Table I Lattice parameters, Wyckoff positions, and Rietveld-refined parameters of NiO, β-Ga₂O₃ and NiGa₂O₄

Figure 2 shows the UV-visible absorption spectra of NiGa₂O₄ in the range of 200–600 nm range. A clear absorption peak is observed at 270 nm. The inset of Fig. 2 shows the Tauc plot [i.e., graph of (αhυ)² versus photon energy (hυ)]. The bandgap energy is estimated by finding an intersection to the energy axis by extrapolating the linear portion of the y-axis. The calculated bandgap energy of the NiGa₂O₄ compound is ~ 4.1 eV.

Fig. 2

The UV-visible absorption spectrum of the NiGa₂O₄ compound. Inset shows the determination of bandgap energy.

Figure 3 shows the feature-rich O K-edge spectra from the Ga₂O₃, NiO, and NiGa₂O₄ compounds. In the case of the O K-edge XANES, the O 1s electrons transition to the empty or hybridized 2p orbitals. In the case of transition metal oxides, there are four types of well-defined partially occupied and unoccupied molecular orbitals. These molecular orbitals, and the corresponding atomic orbitals that contribute dominantly to the molecular orbitals, are in qualitative energetic order of 2t_2g (M 3d; O 2p_π) < 3e_g (M 3d; O 2p_σ) < 3a_1g (M 4s; O 2p_σ) < 4t_1u (M 4p; O 2p_π), where M represent the transition metal element.^23,24 The transition of ls electrons to all four molecular orbitals is dipole-allowed because of the presence of the p-character in these hybridized orbitals.^23,24 A simple molecular orbital theory can be used to understand the main differences in the O K-edge features of various transition metal oxides (Fig. 4).

Fig. 3

O K-edge XANES spectra of NiGa₂O₄ along with reference NiO and β-Ga₂O₃.

Fig. 4

Ni-edge XANES spectra of NiGa₂O₄ and NiO.

In the case of NiO (i.e., Ni²⁺, with 3d⁸ electronic configuration), the 2t_2g orbitals (i.e., group of d_xy, d_xz, and d_yz orbitals, with 6 electrons) are completely occupied, and the degenerate 3e_g orbitals (i.e., group of d_z² and d_x²_−y² orbitals, with 2 electrons) are partially occupied. A sharp O K-edge feature (~ 532.9 eV) in NiO can be assigned to the O 1s electronic transition to the 3e_g orbitals.²³ The other higher-energy features (between 535 eV and 550 eV) are the electronic transition from O 1s to 3a_1g–4t_1u hybridized orbitals. Other multiple scattering features are also present in the O K-edge spectra (above 560 eV) which may originate from electronic transitions from O 1s to hybridized O 2p and metal (n +1) sp orbitals.^23,24 In the case of the Ga₂O₃ compound (β-Ga₂O₃ phase is taken in the present study), the Ga 3d states are filled because of the 3d¹⁰ electronic configuration of Ga³⁺ ions. The final state corresponds to the antibonding orbitals formed by the hybridization of the O 2p states with Ga 4s and Ga 4p.²⁵ It can be seen from Fig. 2 that there is no pre-edge peak (between 528 eV and 532 eV) in the β-Ga₂O₃ reference sample. This is because of the fully occupied 3d orbitals of Ga³⁺ ions. There are two spectral features (between 533 eV and 543 eV) which correspond to the electronic transitions from O 1s to the mixture of O 2p and Ga 4s and Ga 4p states.²⁵ The O K-edge spectrum of the NiGa₂O₄ sample exhibits diverse spectral features. Based on previous reports, the intense pre-edge feature (530.1 eV) may be due to the Ni³⁺ ions present in the NiGa₂O₄ sample as observed for NdNiO₃ samples.²⁵ The O K-edge XANES for NiGa₂O₄ samples is not reported in the literature. Therefore, here we anticipate that the low-energy pre-edge peak in the O K-edge spectrum of the NiGa₂O₄ sample is due to Ni³⁺ ion-related transitions. Additionally, the other features of the NiGa₂O₄ sample partially match with the spectral features of the reference NiO sample. For example, the shoulder-like feature at 532.9 eV coincides with the pre-edge peak of Ni²⁺ ions containing the NiO sample and the high-energy features (at 562.2 eV) of the NiO and NiGa₂O₄ sample also match. This indicates that the NiGa₂O₄ sample is composed of Ni²⁺ and Ni³⁺ ions.

To understand the valance state of Ni in the NiGa₂O₄ compound, Ni K-edge XANES spectra were collected. Figure 3 presents the pre/post-edge background-subtracted and normalized Ni K-edge XANES spectra of the NiGa₂O₄compound. The reference sample of NiO, in which Ni ions obey the +2 valance state, was also scanned under similar experimental conditions, and the spectrum is plotted along with the spectrum of the NiGa₂O₄ compound. Closer examination of the spectral features (see the inset) of the XANES spectra of the NiO and NiGa₂O₄ samples reveals that the rising edge of the NiGa₂O₄ sample is at higher energy. This indicates the existence of a higher valence state of Ni ions (i.e., Ni³⁺) in the NiGa₂O₄ sample. However, the main peak energy of NiO and NiGa₂O₄ samples is fairly coincident and suggests the presence of Ni²⁺ ions. This observation is consistent with the observations of O K-edge spectra where some of the spectral features (the pre-edge peak) were attributed to the Ni³⁺ ions and other features corresponded to the Ni²⁺ ions in the NiGa₂O₄ sample.

To further probe the valence state of the Ga ions in the NiGa₂O₄ compound and to compare the orbital hybridization with the stable oxide of Ga, the Ga K-edge XANES spectrum is collected from the reference β-Ga₂O₃ and Ga₂NiO₄. Ga obeys the 3+ valence state in the β-Ga₂O₃ compound with 50/50 percentage occupancy at tetrahedral/octahedral sites.^24,26 It has also been reported that the α-Ga₂O₃ compound, although the Ga³⁺ ions are solely present in this phase, obeys a nearly 2 eV shift in the white-line peak.^26,27 This is due to the regular octahedral occupancy of Ga³⁺ ions in the α-Ga₂O₃ compound. The normalized Ga K-edge XANES spectra are presented in Fig. 5. It can be seen that the edge-energy positions of the reference β-Ga₂O₃ and NiGa₂O₄ samples coincide with each other, signifying the existence of Ga³⁺ ions in the NiGa₂O₄ sample. The broad or shifted white line peak is observed in the NiGa₂O₄ sample. The shifting/broadening of the white-line peak suggests noticeable octahedral occupancy by the Ga³⁺ ions in the NiGa₂O₄ sample.

Fig. 5

Ga K-edge XANES spectra of NiGa₂O₄ and β-Ga₂O₃.

To understand the unrevealed magnetic properties of the NiGa₂O₄ compound, the M-H data for the sample were collected at two different temperatures (50K and 300K). Figure 6 shows the magnetic hysteresis loop of the NiGa₂O₄ compound. It can be seen from Fig. 6 that the NiGa₂O₄ compound shows paramagnetic behavior at 50 K and 300 K. To understand the elemental origin of the magnetism, it is imperative to elucidate the cationic model of the constituent ions in the NiGa₂O₄ compound. As discussed in the introduction section, the {A_1−xB_x}[B_2−xA_x]O₄ cationic model may exist in the NiGa₂O₄ sample. In this model, the distribution and valence states of the A and B cations (with x = 0) can help us understand the magnetic behavior. From this model, the Ga³⁺ ions may not contribute to the net magnetization because they do not obey unpaired electrons with their fully occupied orbitals (3d¹⁰). Therefore, only the Ni cations are expected to contribute to the observed magnetic properties. Our XANES results may help in validating the above cationic model. The Ga K-edge XANES showed the existence of octahedrally distributed Ga³⁺ ions in the NiGa₂O₄ compound. Likewise, Ni²⁺ ions (majority) and Ni³⁺ ions (minority) were observed in the Ni K-edge XANES. The Ni²⁺ ions may provide NiO-type antiferromagnetic ordering in the sample. However, the Ni³⁺ ions with 3d⁷ electronic configuration may contribute to the paramagnetic character of the NiGa₂O₄ compound. The magnetic characterization of NiGa₂O₄ is less explored in the literature. Therefore, further theoretical and experimental studies are needed to confirm the origin of magnetism in NiGa₂O₄. It is seen in Fig. 6 that the magnetization is decreased at 300 K. The magnetization can randomly flip direction under the influence of temperature.²⁸ At low temperatures, the thermal agitation is minimized and the degree of alignment of the atomic magnetic moments is the greatest. At high temperatures, abundant thermal agitation leads to a decrease in the alignment of the atomic magnetic moments, and consequently the magnetization decreases.

Fig. 6

Magnetic field versus magnetization data for NiGa₂O₄, measured at 50 K and 300 K.

Conclusions

A NiGa₂O₄ compound is prepared using a sonication-assisted chemical preparation method and subsequent annealing at 1150°C. Rietveld refinement is performed on the XRD patterns and confirmed the single-phase nature with lattice parameters of a = b = c= 8.2589 Å and α = β = γ = 90°. The bandgap energy of the NiGa₂O₄ compound is ~4.1 eV. The O K-edge, Ni K-edge, and Ga K-edge XANES results confirm the Ni²⁺/Ni³⁺ ions and Ga³⁺ ion in the NiGa₂O₄ compound. Magnetic studies (at 50 K) confirm the paramagnetic character of the compound. The magnetization decreases at 300 K because of a decrease in the alignment of the atomic magnetic moments at higher temperature.